This invention relates to a process for the manufacture of formaldehyde by the oxidative dehydrogenation of methanol in the presence of a silver or copper catalyst. More particularly this invention relates to a process for the manufacture of formaldehyde by the oxidative dehydrogenation of methanol in the presence of a silver or copper catalyst in which the aqueous formaldehyde product is stripped of methanol and water by means of the off-gas stream.
In the industrial manufacture of formaldehyde from methanol by dehydrogenating oxidation with air over silver or copper catalyst in the presence of steam, the formaldehyde is usually washed out of or scrubbed from the reaction gas with water. The starting mixture in general contains methanol (calculated as 100% strength) and water in a ratio of from 0.1 to 1.8 moles of water per mole of methanol. On absorption of the reaction mixture, the steam produced by the reaction and the steam and methanol left from the starting mixture are condensed. The formaldehyde combines with water to give methylene glycol and higher polyoxymethylene glycols and with residual methanol to give methyl hemiformal and polyoxymethylene glycol monomethyl ethers. The higher polyoxymethylene glycols tend to precipitate from concentrated aqueous formaldehyde solutions as paraform. Hence aqueous formaldehyde has been conveniently used at a concentration of about 30 to about 37% by weight since such solutions are stable over extended periods of time without precipitation of paraformaldehyde and have been conveniently used in the manufacture of phenolic and amino resins.
In more recent times, with the development of improved stabilizers to suppress paraformaldehyde formation, higher concentrations of aqueous formaldehyde have been accepted for the handling of formaldehyde and the manufacture of resins and have provided energy savings since they improve shipping and handling efficiency and reduce the amount of water to be removed from the resin products. Such concentrated solutions are generally prepared by distilling the aqueous formaldehyde solutions formed in absorbers under conditions which allow most of the unconverted methanol to be removed. The distillation step requires high reflux ratios and considerable energy input.
The present invention is an improved process for the preparation of aqueous formaldehyde solutions in which methanol and water are removed from aqueous formaldehyde solution formed in the absorber, by stripping the solution with off-gas. The solution can be maintained at an elevated temperature during the stripping operation with heat generated in the oxidative-dehydrogenation of the methanol starting material. In contrast with the high reflux ratios required in the removal of methanol and water by distillation, little or no reflux is required and considerable improvement in energy efficiency of the process is realized. By means of the stripping step, a concentrated aqueous formaldehyde solution of low methanol content can be readily obtained and can be used advantageously in the manufacture of phenolic and amino resins and particularly of C.sub.2 and higher alkylated amino resins. Methanol contents of less than 2 weight percent are readily obtained.
In summary the invention is a process for the manufacture of an aqueous solution of formaldehyde comprising the steps of:
(a) oxidatively dehydrogenating methanol with air in the presence of a silver or copper catalyst and steam at elevated temperature;
(b) absorbing the reaction product in an absorber comprising one or more absorption stages in series to form an aqueous formaldehyde solution containing free and combined methanol; and
(c) stripping the methanol from the aqueous formaldehyde solution with the off-gas stream which emerges from the absorber, by countercurrent flow in a stripping column comprising at least about 1.5 theoretical transfer units for methanol stripping, the stripping temperature and the ratio of stripping gas to aqueous formaldehyde being selected to provide a concentration of vapors of aqueous formaldehyde in the gas emerging from the stripping column of no more than about 50 mol percent.
Essentially the process is advantageously carried out continuously in the following sequence:
1. a mixture of water and methanol vapors is mixed with air, the mixture is passed over a silver or copper catalyst bed and the methanol is oxidatively dehydrogenated;
2. the reaction product comprising a gaseous mixture of formaldehyde, steam, residual methanol, nitrogen, carbon dioxide and hydrogen is cooled and fed to an absorber comprising a series of stages containing aqueous formaldehyde solution, each stage being equipped with a circulation loop and a cooling system. Sufficient water or dilute aqueous formaldehyde solution is added to the final absorber stage to maintain the desired concentration of formaldehyde in the final product, and aqueous formaldehyde solution is passed through the absorber countercurrently to the reaction gas mixture to absorb most of the formaldehyde, water and methanol from the gas mixture;
3. aqueous formaldehyde solution is fed from the absorber to a multi-stage stripping column while, countercurrently to the aqueous formaldehyde flowing in the multi-stage stripping column, a stream of the off-gas which emerges from the top of the absorber is circulated to strip some of the residual methanol from the solution and simultaneously some water and formaldehyde;
4. the gas stream which emerges from the stripping column is treated to recover most of the stripped methanol, water and formaldehyde;
5. the aqueous formaldehyde solution which is drawn from the bottom of the stripping column constitutes the final formalin product from the process.
The operation of the stripping column is dependent on several parameters including temperature of the column, ratio of stripping off-gas to aqueous formaldehyde solution, the number of ideal stages or theoretical transfer units in the column, the residence time of the aqueous formaldehyde solution and the number of aqueous formaldehyde streams supplied to the stripping column from the absorber.
The temperature of the stripping column can be any temperature from atmospheric temperature up to the temperature at which the partial pressure of the vaporized aqueous formaldehyde under the operating conditions of the column is no more than about 50 percent of the total gas pressure of the gas leaving the top of the column or in other words the temperature at which the gas leaving the top of the column contains no more than about 50 mol percent of vapors of the aqueous formaldehyde. Preferably the temperature is selected so that, under the operating conditions of the column, the gas leaving the top of the column comprises about 20 to about 40 mol percent of vapors of the aqueous formaldehyde. Advantageously the column is operated under conditions such that the temperature of the column is in the range of about 60.degree. to about 85.degree. C. and more preferably in the range of about 65.degree. to about 80.degree. C. Similarly the weight ratio of stripping off-gas to aqueous formaldehyde solution passed through the column per unit time is selected so that under the operating conditions of the column the gas mixture leaving the column contains no more than about 50 mol percent of vapors of aqueous formaldehyde and preferably contains from about 20 to about 40 mol percent. In practice, the off-gas stream in the stripping column is advantageously maintained at a pressure above atmospheric pressure, preferably in the range of about 1.01 to about 2 atmospheres and the ratio of gas entering the stripping column to aqueous formaldehyde solution added to the stripping column from the first absorption stage is advantageously in the range of about 0.5 to about 2.5 by weight per unit time and is preferably in the range of about 1.0 to about 2.0.
In general the absorber can contain any number of absorption or scrubber stages for absorption of the reaction products of the oxidative dehydrogenation of methanol. Conveniently from 2 to 4 absorption stages each equipped with a recirculation loop and cooling system may be used. Sufficient water is continuously added to the system to maintain the desired formalin product concentration, and the temperatures and concentrations of the aqueous formaldehyde solutions in the stages are preferably maintained at levels for efficient absorption of formaldehyde and methanol from the reaction gas stream without formation of paraform in the stages. Part of the circulating stream of at least the first absorption stage is passed to the stripping column. Preferably none of the aqueous formaldehyde solution passed to the stripping column from the first absorption stage is returned to the absorber. It is preferably taken off at the bottom of the stripping column as formalin product. Preferably part of the recirculating streams of at least the first two absorption stages are passed to the stripping column, the points of entry being intermediate to top and bottom of the stripping column with the more dilute aqueous formaldehyde solution entering near the top of the column and the more concentrated solution entering near the bottom while at the same time the more dilute aqueous formaldehyde solution after descent in the stripping column is partly drawn off as a side stream above the entry point for the more concentrated solution and added to the circulation loop of the prior more concentrated aqueous formaldehyde absorption stage to maintain the concentration of formaldehyde in that absorption stage at a level to avoid formation of paraform at the particular temperature of the stage. Where part of the aqueous formaldehyde solution in an absorption stage other than the first absorption stage is supplied to the stripping column, the side stream from the stripping column may provide the only route whereby formaldehyde solution from that absorption stage can flow to the prior absorption stage containing more concentrated aqueous formaldehyde. While the stripper column preferably has at least two formaldehyde streams entering it from the absorber it can have as many entering streams as there are absorption stages in the absorber, the streams providing a formaldehyde concentration gradient in the stripping column.
In order to obtain significant removal of methanol from the aqueous formaldehyde by means of the stripping column without excessive removal of formaldehyde, the stripping column should comprise at least about 1.5 theoretical transfer units and preferably about 3 or more theoretical transfer units. The number of theoretical transfer units is determined from the following relationship: EQU NTU=(.DELTA.P.sub.t -.DELTA.P.sub.b)/[1/2(P.sub.t +P.sub.b)]
where
NTU=number of transfer units in the stripping column under steady state conditions, PA1 P.sub.t =vapor pressure of methanol in the gas stream emerging from the top of the stripping column, PA1 P.sub.b =vapor pressure of methanol in the gas stream entering the bottom of the stripping column, PA1 .DELTA.P.sub.t =P.sub.t (e)-P.sub.t, PA1 P.sub.t (e)=equilibrium vapor pressure of methanol for the aqueous formaldehyde/methanol solution at the top of the column, at the temperature at the top of the column, PA1 .DELTA.P.sub.b =P.sub.b (e)-P.sub.b, and PA1 P.sub.b (e)=equilibrium vapor pressure of methanol for the aqueous formaldehyde/methanol solution at the bottom of the column, at the temperature at the bottom of the column.
The equilibrium vapor pressures are determined from standard vapor liquid equilibrium data, for example they may be obtained from data stored in the data base sold by Monsanto under the registered trademark Flowtran.
In aqueous formaldehyde solutions a methanol-methyl hemiformal-polyoxymethylene glycol monomethyl ether equilibrium exists and favors the hemiformals. The equilibrium can be displaced towards methanol by raising the temperature and by dilution of the formaldehyde solution with water. Methanol can be readily removed from dilute aqueous formaldehyde solutions by fractional distillation. However with concentrated solutions of aqueous formaldehyde which are gaining commercial favor, wherein the formaldehyde concentration is above about 40 weight percent and particularly wherein the formaldehyde concentration is above about 50 weight percent, processes to remove methanol and in particular the stripping process of the present invention to remove methanol at the relatively low temperatures used with the purpose of conserving energy, is more difficult because most of the methanol is chemically combined as hemiformals at these temperatures and, although reversal of the methanol hemiformal reaction occurs progressively with the removal of methanol from the solutions, the rate of reversal is rather low. Efficient removal of methanol in a stripping column comprising conventional packaging or tray columns would require an excessively high column. It is therefore advantageous to increase the residence time of the aqueous formaldehyde in the stripping column by any convenient means to allow equilibrium reversal of the methanol hemiformal reaction to occur. Preferably residence zones are introduced to allow the aqueous formaldehyde solution to remain quiescent out of contact with the stripping off-gas generally for at least about 4 minutes until a significant concentration of free methanol has been established. The column then becomes a series of stripping zones separated by residence zones, with the free methanol being generated by reversal of the methanol hemiformal reaction in the residence zones. One way to obtain quiescent residence zones is by introduction into the column of a number of chimney trays which are essentially overflow liquid trays with gas chimneys to allow the stripping gas ascending to pass by the aqueous formaldehyde solution held in the chimney trays. Another way is by means of circulation loops placed at intervals along the stripping column, the loops being equipped with reservoirs of suitable size to isolate the formaldehyde solution from the gas stream for the desired time. Thus with stripping columns comprising conventional packing or trays such as sieve trays, glass trays, bubble-cap trays or valve trays to provide intimate contact between the aqueous formaldehyde solution and the stripping off-gas for efficient extraction of methanol by the gas stream, it is advantageous to include residence zones at intervals in the column to reduce the height of the column required for efficient stripping of methanol. For example a 30 meter stripping column capable of handling about 5 metric tons of formalin product per hour, provides about four theoretical transfer units determined by means of the relationship set forth above, for the stripping of methanol when it is packed with 21 meters of Pall ring packing divided into 5 zones with each zone separated with a chimney tray of 10 cm. depth, providing a residence time of about 6 minutes in each residence tray. Similarly a 30 meter stripping column containing 45 sieve trays can provide about four theoretical transfer units when 4 residence trays each providing a residence time of about 6 minutes are placed at intervals along the column. Thus by means of the residence zones, transfer units of height in the range of about 1 to about 10 meters are readily obtained and allow the desired weight ratio of gas to liquid passing through the stripping column per unit time to be achieved.
The stripping gas which emerges from the stripping column containing methanol, formaldehyde and water vapors also contains carbon dioxide and hydrogen produced in the reactor and carried throughout the absorber. The gas is advantageously treated to remove the condensible components for example by scrubbing the gas with a cooled recirculated solution of aqueous formaldehyde to which water is constantly added and from which a portion is constantly taken off and passed to the circulation loop of the last absorption stage of the absorber. The amount of water added is adjusted to maintain the desired concentration of water throughout the entire system and to provide for the desired concentration of aqueous formaldehyde product. The treated gas can then be passed through a condenser to form an aqueous formaldehyde-methanol solution which is relatively rich in methanol and can be advantageously volatilized and recirculated to the methanol reactor. Because a major portion of the methanol present in the aqueous formaldehyde solution in the stripper column can be advantageously removed by the stripping action, the aqueous formaldehyde-methanol solution recycled to the reactor is characterized by a methanol to formaldehyde mol ratio of at least about 0.25 and a ratio of at least about 0.45 can be readily achieved. The mol ratio is generally at least about 10 times higher than the ratio of the starting solution. The off-gas stream emerging from the condenser is passed to an incinerator to burn the hydrogen gas present in it and recover its fuel value.
Optionally the off-gas which emerges from the stripping column can be passed to a partial condenser and the condensate can be returned to the top of the stripping column. In this manner, most of the formaldehyde and water is condensed and returned as a reflux to the stripping column while the off-gas stream emerging from the partial condenser retains most of the methanol removed from the aqueous formaldehyde solution in the stripping column. When the aqueous formaldehyde condensate is refluxed to the stripping column in this manner, the stripping column can advantageously be equipped with a top stage comprising a contact zone for intimate contact between the ascending inert gas stream and the descending aqueous formaldehyde reflux and a residence zone above the entry port for the most dilute aqueous formaldehyde solution entering the stripping column from the absorber. The off-gas stream emerging from the partial condenser is passed through a condenser and the condensate comprising mostly methanol can be volatilized and added to the reaction gas stream. The methanol condensate can advantageously have a methanol to formaldehyde mol ratio of at least about 0.6 and a ratio of at least about 1.0 can be readily achieved, and the mol ratio of recirculated formaldehyde to total methanol in the reaction gas stream can advantageously be less than about 0.035 and indeed less than about 0.03 to avoid unnecessary recycling of formaldehyde.
In comparison with a fractional distillation column for removal of methanol from the aqueous formaldehyde solution produced in the absorber, the gas-stripping process of the present invention can reduce the energy requirement for methanol removal by about 50 percent or more without sacrifice in separating efficiency. Advantageously for a balance in energy savings and separating efficiency, the stripping column is maintained at a temperature in the range of about 60.degree. to about 85.degree. C., and more preferably in the range of about 65.degree. to about 80.degree. C. and the ratio of stripping gas fed into the bottom of the stripping column to the most concentrated aqueous formaldehyde solution fed to the lower part of the stripping column from the absorber is in the range of about 0.5 to about 2.5 by weight per unit of time and more preferably in the range of about 1.0 to about 2.0.
Since the stripping column is operated at a relatively low temperature and since the reflux returned to the stripping column is a very minor fraction of the total amount of aqueous formaldehyde solution in the stripping column, the heat required to maintain the stripping column at the operating temperature can be readily supplied by the absorption solution from the first stage of the absorber and no external source of heat may be required.